1. Field of the Invention
This invention relates to recovery of hydrocarbons from sulfones formed by oxidative desulfurization.
2. Description of Related Art
Conventional hydrodesulfurization (HDS) processes are based on catalytic hydrogenation conducted at a relatively high pressure (about 30 bars to about 80 bars) and temperature (about 270° C. to about 330° C.). Sulfur compounds can be classified into four groups according to their HDS reactivity described by the pseudo-first-order rate constants. See, e.g., X. Ma, K. Sakanishi and I. Mochida, Hydrodesulfurization reactivities of various sulfur compounds in diesel fuel. Ind. Eng. Chem., 1994, 33, 218; X. Ma, K. Sakanishi, T. Isoda and I. Mochida, Hydrodesulfurization reactivities of narrow-cut fractions in a gas oil. Ind. Eng. Chem. Res., 1995, 34, 748. These groups are:
The first group is predominantly alkyl benzothiophenes (BTs); the second, dibenzothiophenes (DBTs) and alkyl DBTs without alkyl substituents at the 4- and 6-positions; the third group, alkyl DBTs with only one alkyl substituent at either the 4- or 6-position; the fourth group, alkyl DBTs with alkyl substituents at the 4- and 6-positions. The sulfur content in the four groups in the is 39, 20, 26 and 15 wt. %, respectively. The relative HDS rate constant for each of the four groups is 36, 8, 3, and 1, respectively.
When the total sulfur content is reduced to 500 ppmw, the main sulfur compounds remaining in the hydrotreated effluent are the third and fourth groups. When the total sulfur content is reduced to 30 ppmw, the sulfur compounds remaining are only the fourth group sulfur compounds, indicating that the lower sulfur content organosulfur compounds have lower HDS reactivity. See D. D. Whitehurst, H. Farag, T. Nagamatsu, K. Sakanishi and I. Mochida, Assessment of limitations and potentials for improvement in deep desulfurization through detailed kinetic analysis of mechanistic pathways. Catalysis. Today, 1998, 45, 299. Additional studies using various straight-run gas oils from different crude oils confirmed the differences in reactivity between different sulfur compounds. See, e.g., J. A. R. van Veen and S. T. Sie, Deep hydrodesulfurization of diesel fuel. Fuel Process. Technol., 1999, 61, 1; H. Schulz, W. Bohringer, F. Ousmanov and F. Waller, Refractory sulfur compounds in gas oils. Fuel Process. Technol. 1999, 61, 5.
Further investigations have demonstrated that sulfur compounds remaining in diesel fuels at sulfur level lower than 500 ppmw are dibenzothiophenes with alkyl substituents at the 4- and/or 6-position, and are lower in HDS reactivity. See, e.g., Ma Ind. Eng. Chem., 1994, 33, 218; T. Kabe, A. Ishihara and H. Tajima, Hydrodesulfurization of sulfur-containing polyaromatic compounds in light oil. Ind. Eng. Chem. Res., 1992, 31, 1577; X. Ma, K. Sakanishi and I. Mochida, Hydrodesulfurization reactivites of various sulfur compounds in vacuum gas oil. Ind. Eng. Chem. Res., 1996, 35, 2487; B. C. Gates and H. Topsoe, Reactivities in deep catalytic hydrodesulfurization: challenges, opportunities, and the importance of 4-methyldibenzothiophene and 4,6-dimethyl-dibenzothiophene. Polyhedron, 1997, 16, 3213; X. Ma, Deep hydrodesulfurization of diesel fuel: chemistry and reaction processing design, Ph.D. Thesis, Kyushu University, Japan, 1995; X. Ma, K. Sakanishi, T. Isoda, I. Mochida, Comparison of Sulfided CoMo/Al2O3 and NiMo/Al2O3 catalysts, in: M. L. Occelli, R. Chianelli (Eds.), Hydrodesulfurization of Gas Oil Fractions and Model Compounds, in Hydrotreating Technology for Pollution Control, Marcel Dekker, New York, 1996, 183. Consequently, these species are referred to as refractory sulfur compounds. Both steric hindrance and electronic density factor contribute to the observed low reactivity levels of 4- and 6-substituted DBTs in HDS process. See X. Ma, K. Sakanishi, T. Isoda and I. Mochida, Quantum chemical calculation on the desulfurization reactivities of heterocyclic sulfur compounds. Energy Fuels, 1995, 9, 33; M. Daage and R. R. Chianelli, Structure-function relations in molybdenum sulfide catalysts: the rim-edge model. J. Catal., 1994, 194, 414.
Due to the low reactivity of refractory sulfur compounds, HDS can normally reduce the crude oil sulfur content from a few thousand ppmw to nearly 500 ppmw. However, it is not economically viable to remove the remainder of the sulfur content due to the high temperature and pressure requirements.
Compared with conventional catalytic HDS, oxidative desulfurization (ODS) can be performed under mild conditions, i.e., relatively low temperature and under atmospheric pressure conditions. ODS typically uses an oxidizing agent, such as hydrogen peroxide, organic peroxide, peracid and ozone, in addition to an oxidation catalyst. In the oxidation process, the divalent sulfur atom of refractory sulfur compounds (condensed thiophene) is oxidized by the electrophilic addition reaction of oxygen atoms to form the hexavalent sulfur of sulfones. The chemical and physical properties of sulfones are significantly different from those of the hydrocarbons in fuel oil. Therefore, sulfones can be removed by conventional separation methods such as filtration, solvent extraction and adsorption. An effective ODS process, which can decrease sulfur in the transportation fuel from 1100 ppm to 40 ppmw, is described in WO/2007/103440 filed on Mar. 5, 2007 (F. Al-Shahrani, T. Xiao, G. D. Martinie and M. L. H. Green, Catalytic Process For Deep Oxidative Desulfurization of Liquid Transportation Fuels) and in F. Al-Shahrani, T. Xiao, S. A. Llewellyn, S. Barri, Z. Jiang, H. Shi, G. Martinie and M. L. H. Green, Applied Catalysis B, V. 73., No. 3-4, p. 311 (2007). ODS is considered a promising substitute or supplement to HDS for deep desulfurization of transportation fuels.
The compositions of common sulfides in fuel oil and their respective sulfones are tabulated in Table 1:
TABLE 14-4-4,6-4,6-DBTDBTO2MDBTMDBTO2DMDBTDMDBTO2C H %82.5870.3683.8172.1684.8873.76S %17.4214.8416.1913.9415.1213.14O %014.80013.90013.10
Sulfides consist of carbon, hydrogen and sulfur. For example, DBT is constituted by 82.58% carbon and hydrogen (hydrocarbon) and 17.42% sulfur. Sulfone consists of carbon, hydrogen, sulfur and oxygen. For example, DBT sulfone is constituted by 70.36% hydrocarbon, 14.84% sulfur and 14.80% oxygen. With alkyl substituted DBT sulfone, the percentage of hydrocarbon increases. For example, hydrocarbons constitute 72.16% of MDBT sulfone and 73.76% of DMDBT sulfone. Sulfones formed by ODS processes from diesel fuel are not a single species, but a very complicated mixture which includes not only DBT sulfone, but also several alkyl substituted DBT sulfones, such as 4-MDBT sulfone, 4,6-DMDBT sulfone, 1,4-DMDBT sulfone, 1,3-DMDBT sulfone, TriMDBT sulfone, TriEDBT sulfone, and C3 DBT sulfone. The structures of these sulfones are given below. The sulfone species may vary with different source of diesel.

The GC-MS of mixed sulfones from diesel fuel was reported in M. F. Ali, A. Al-Malki, B. El-Ali, G. Martinie and M. N. Siddiqui, Fuel, 2006, 85, 1354, and is presented in FIG. 1. From this data, it is clear that (1) the sulfones from diesel fuel are an extremely complicated mixture; (2) most of the sulfones are alkyl substituted DBT sulfones; (3) the highest percentage is 4,6-DMDBT sulfone; (4) non-substituted DBT sulfone is only a very small percentage; and (5) there remain some alkyl substituted DBT sulfones that are difficult to completely identify.
Unlike HDS, in which hydrogenated products remain with fuel oil and organic sulfur is converted into gaseous H2S that leave the fuel oil mixture, sulfones formed by ODS must be separated and removed. Since hydrocarbons constitute more than 70% of a sulfone compound, separation and removal of sulfone will inevitably cause hydrocarbon yield loss in the fuel oil product and generation of solid waste. Generation of 1 g of sulfone will cause a loss of more than 0.7 g of hydrocarbon from. In an ODS process, for 1 million tons of diesel containing 500 ppme sulfur, based on DBT only, 2870 tons of DBT will be lost and 3368 tons of DBT sulfone will be generated. If the calculation is based on only 4-MDBT 3088 tons of 4-MDBT (0.31%) will be lost and 3586 tons of 4-MDBT sulfone will be generated. If the calculation is based on only 4,6-DMDBT, the loss of hydrocarbon and generation of sulfone will both increase. Table 2 details these calculations for the loss of hydrocarbon and generation of sulfones based on 1 million tons of diesel containing 500 ppme sulfur.
TABLE 24-4-4,6-4,6-NameDieselSDBTDBTO2MDBTMDBTO2DMDBTDMDBTO2Amount  106500287033683088358633073805(tons)%1000.050.290.340.310.360.330.38
Therefore, recovery of hydrocarbons from sulfones generated by ODS is an important step to reduce hydrocarbon yield loss and to avoid the increased cost of solid waste disposal. Furthermore, recovery of hydrocarbons from sulfones generated by ODS can enhance the desirability of using ODS for oil refining.
Various attempts have been made to recover hydrocarbons from DBT sulfone. These include pyrolysis, decomposition in the presence of alkali in an organic solvent, decomposition in the presence of alkali in water, and decomposition in the presence of potassium fluoride in the presence of supercritical water.
Direct decomposition of dibenzothiophene sulfone was studied by Fields and Meyerson (E. K. Fields and S. Meyerson, J. Am. Chem. Soc., 1966, 88, 2836). Pyrolysis of DBT sulfone was conducted at 690° C. with a contact time of 15 seconds was reported provide a 95% yield of a 6:1 mixture of dibenzofuran and dibenzothiophene:
Wallace and Heimlich (T. J. Wallace and B. N. Heimlich, Tetrahedron, 1968, 24, 1311) studied the mechanism of reaction for alkali decomposition of DBT sulfone and related compounds in white oil as an organic solvent. The results of alkali decomposition indicated that the stability of the DBT nucleus is markedly dependent on the oxidation state of the S-atom. The products formed in the decomposition reaction vary with temperature, contact time, and the initial ratio of base to dioxide. They observed the formation of 18% sodium-2-phenylbenzenesulfonate, 5.8% sodium-2-phenylphenolate, 2% biphenyl and 19.5% dibenzofuran when DBT sulfone was treated with sodium hydroxide in white oil at 300° C. for 4 h. After a similar treatment for 5.5 h, only sulfur-free products, 5% biphenyl and >90% dibenzofuran, were observed:

Table 3 shows the of decomposition of DBT sulfone under various reaction conditions in white oil.
TABLE 3Products, mole % yield    BaseBase/ DBT sul- fone (mol)   Temp. ° C.    Time/h   KOH5200 345.040.05.0—KOH5250 1.747.650.71.03.0KOH5300⅙70.029.61.70.8NaOH5300 5.5——>90~5NaOH5300 418.058.019.52.0None—30023No reaction
Lacourt and Friedman (R. B. Lacount and S. Friedman, J. Org. Chem., 1977, 42, 2751) reported the decomposition reaction of DBT sulfone in excess aqueous alkali (NaOH) at 300° C. in an autoclave. After acidification, 2-phenylphenol was obtained as the only organic compound and sodium sulfite was confirmed in the water layer as shown below:
Calcium oxide and sodium carbonate were also used for comparison, as shown in Table 4.
TABLE 4Base/DBTTemp.Time/2-SulfoneBasesulfone(mol)° C.hphenylphenol, %recovered %NaOH530051000NaOH53001990NaOH52001489CaO530051576Na2CO353005890
Varga et al (T. R. Varga, Y. Ikeda and H. Tomiyasu, Energy & Fuels, 2004, 18, 287) reported that hydrocarbon recovery can be accomplished by reaction of sulfones and KF in supercritical water as shown below:

Table 5 below summarizes the researches for hydrocarbon recovery from DBT sulfone as a model compound. There are only a few examples available in the literature.
TABLE 5Summary of research for hydrocarbon recovery from DBT sulfoneReactionReactantBaseconditionProductsNotesAuthorDBTO2nonePyrolysis 690° C.Desulfur incomplete, temperature too high to realizeField & Meyerson DBTO2NaOH300° C. 5.5 h in white oilDesulfur completeWallace & Heimlich DBTO2NaOH300° C. 5 h in water in autoclaveDesulfur completeLacourt & Friedman DBTO2KF380° C. in autoclave, supercritical waterDesulfur complete, condition too harsh to realizeVarga et al
Table 5 indicates that the recovery products are mainly dependent on reaction condition. For pyrolysis, the temperature was up to 690° C. Beside the sulfur-free product dibenzofuran, there was greater than 15% DBT. Further, the very high temperature limits the applicability of this process. For decomposition in white oil, the reaction was carried out under nitrogen at 300° C. There were two different sulfur-free products, dibenzofuran and biphenyl. While there is no requirement for an autoclave, there are few organic solvents that can withstand operating temperatures of 300° C. For decomposition in water, the reaction was run in an autoclave at 300° C. There was only one sulfur-free product, [1,1′-biphenyl]-2-ol/or 2-phenylphenol. For decomposition under supercritical water conditions, the reaction was run in an autoclave at 380° C. The only product was [1,1′-biphenyl]-2-ol/or 2-phenylphenol.
A major concern with the above-described existing approaches is that hydrocarbon recovery results were based on the commercially available DBT sulfone. However, hydrocarbon recovery from the substituted DBT sulfones or mixtures of these sulfones has not been reported. There is also no report of hydrocarbon recovery from sulfones formed from fuel oil. As shown in FIG. 1, and as discussed above and in the referenced F. Al-Shahrani et al. PCT application, the F. Al-Shahrani 2007 article and the M. F. Ali 2006 article, sulfones formed by ODS of diesel are an extremely complicated mixture, and DBT sulfone represents only a very small percentage of this mixture. Therefore, it is inappropriate to use solely DBT sulfone as the model compound in a study of hydrocarbon recovery from the mixed sulfones formed by ODS. Substituted DBT sulfones are not commercially available alone or as a mixture.